Tutorials:
http://www.regular-expressions.info/tutorial.html - One great tutorial on regex
Online Testers:
http://osteele.com/tools/reanimator/ - it animates your regex as an automaton, cool!
http://regexpal.com/ - really cool and easy to use
http://www.fileformat.info/tool/regex.htm
Tools:
http://www.radsoftware.com.au/regexdesigner/
.Net specific:
http://msdn.microsoft.com/en-us/library/az24scfc(VS.71).aspx
Java specific:
http://java.sun.com/docs/books/tutorial/essential/regex/
GeneXus X specific:
http://wiki.gxtechnical.com/commwiki/servlet/hwikibypageid?4606